This invention relates to a method for reducing the number of defects in a film structure resulting from deposition of minute particles on the substrate before, during, or after a fabrication process. Such defects are of particular importance for the production of integrated circuits having both small feature size and large numbers of components. One example of such integrated circuits is very high capacity (&gt;1 Mbyte) memory chips. Even though some redundancy can be designed into such a circuit, the probability that a small number of minute defects will inactivate the circuit is high. The present invention seeks to address such process limitations.
In semiconductor manufacturing, many process steps take place at pressures well below atmospheric pressure. As a result, the pressure surrounding the semiconductor substrates (wafers) being processed must be reduced to at least the working pressure of the process steps in preparation for processing. During processing, a combination of, e.g., chemical vapor and physical deposition and etching processes may require the pressure of the processing environment to repeatedly cycle between a fraction of atmospheric pressure and nearly zero pressure. Following processing, the wafers are returned to atmospheric pressure so they can be removed from the film growth/etching processing chamber and prepared for further processing steps.
Considerable evidence exists suggesting that altering the pressure surrounding the wafers in the course of fabrication results in deposition of microscopic particles on the wafers. The most likely mechanism for this effect has to do with differential changes of wafer, gas, and chamber temperatures which occur during changes of gas pressure.
A wafer suspended in a chamber (usually mounted on a paddle with pins or in a boat via an edge contact) is typically not in good thermal contact with the surrounding chamber walls. (Reference to a chamber or a fabrication chamber is intended also to refer to any vessel utilized in fabrication processes taking place at sub-atmospheric pressures, e.g., a loadlock.) As the surrounding gas is pumped away, the temperature of the gas remaining in the chamber decreases. Both the wafer and the chamber walls lose heat to the gas, and are thereby cooled. As the wafer is nearly isolated from any source of heat but the gas, and has very small thermal mass, the temperature of the wafer decreases considerably more than does the chamber in response to the cooling of the circumambient atmosphere. When pumping is finished, contact with the chamber serves to heat the remaining gas to the desired process temperature. As the gas temperature rises above the wafer temperature, the wafer will begin to heat slowly, leaving the wafer temperature significantly below both the gas and chamber temperatures for a period of time. During the inverse process (increasing pressure), compressive heating of the gas also results in a period of time when the wafer temperature is less than the gas temperature.
Considerations based on molecular transport physics suggest that there is an overall tendency for particles and other contaminants suspended in a medium having a thermal gradient to be driven preferentially toward cooler temperatures. (This effect is called thermophoresis.) When the wafer is colder than the surrounding gas (for the purposes of this application, the term "surrounding gas" refers to the gas which contributes to the thermophoretic force at the surface of the wafer), the resulting temperature gradient in the gas surrounding the wafer produces thermophoretic forces which drive the process of particle deposition on the wafer. Pressure changes which occur in the normal course of fabrication can drive heating or cooling of the gas in a chamber, thereby creating such temperature gradients.
The velocity of particles driven by thermophoresis in the low pressure (sub-atsmospheric) regime is proportional to the temperature gradient and the gas viscosity, and inversely proportional to the square root of the gas molecular weight. Reduction of particle deposition on wafers during fabrication processes can thus be effected by taking measures to insure that the wafer temperature remains above the temperature of the surrounding gas, especially during pressure changes. Such techniques can be supplemented by other measures, such as cooling the chamber walls to a temperature below that of the wafer so as to cool the gas filling the chamber and to preferentially trap particles on the chamber walls, controlling the dynamics of any change in gas pressure to minimize the differential temperature between the wafer and the surrounding gas, and, on venting the chamber, precooling the incoming gas to offset pressurization-related heating of the gas in the chamber. However, the essence of the technique is to maintain the wafer at a higher temperature than the surrounding gas during changes in gas pressure.
The nearest prior art is probably in U.S. Pat. No. 5,373,806 (Logar). Logar describes a method to reduce or eliminate particles and particle-generated defects in gas-phase processing. His emphasis is on removing the effects of electrostatic attraction between particles and a surface through the use of IR radiation, thermal heating, or inductive heating to above about 180.degree. C., but below the intended process temperature. Logar also states that the primary effectiveness of his method occurs during purging the chamber with cold gas.
The present method requires controlling process parameters so that the gas surrounding the wafer will never be at a higher temperature than the wafer surface.
In the case of initial evacuation from ambient atmospheric conditions (nominally STP (standard temperature and pressure)), the wafer temperature required to implement the present invention is only perhaps 30.degree. C., a value far short of the 180.degree. C. taught by Logar. In the case of venting the chamber, however, heating of the wafer to a temperature 10-20.degree. C. higher than the process temperature will typically be required to carry out the present method, whereas temperatures above the process temperature are specifically ruled out by Logar. Accordingly, the present method and that of Logar are clearly disjoint, although they may be carried out by rather similar apparatus.
An object of the present invention is to provide a method for reduction of particle deposition on wafers during fabrication processes involving changes in gas pressure within the fabrication chamber.
Another object of the present invention is to detail apparatus which enables the above method and is easy to retrofit to existing fabrication chambers (including access loadlocks as are well known in the art).
A further object of the present invention is to minimize deposition of particles on selected surfaces within a chamber undergoing changes in pressure.
These and other objects of the present invention will be made clear in the following discussion.